1. Technical Field
The present invention relates to a light source device, a projector, and a method of driving a discharge lamp.
2. Related Art
As a light source of a projector, there is used a discharge lamp such as a high-pressure mercury lamp or a metal halide lamp.
Such a discharge lamp is driven with a drive method of, for example, supplying a high-frequency alternating current as a drive current (see, e.g., JP-A-2007-115534 (Document 1)). According to this drive method, stability of discharge can be obtained, blackening and devitrification of the discharge lamp main body can be prevented, and the degradation of the lifetime of the discharge lamp can be suppressed.
However, in this drive method, the arc discharge occurs between a pair of electrodes while the discharge lamp is in the lighting state, and the electrodes are heated to a high temperature, and therefore, the electrodes are melted, and the distance between the electrodes grows.
In the usage of, for example, a projector, it is preferable for improving the efficiency of the light to keep the state in which the distance between the electrodes is short to thereby reduce the size of the light emission. The growing of the distance between the electrodes during lighting leads to the degradation of the efficiency of the light, which is not preferable.
On the other hand, there also exists a drive method of supplying an alternating current (an alternating direct current) having a low frequency and a rectangular waveform as the drive current (see, e.g., JP-A-2010-114064 (Document 2)). According to this drive method, a projection is provided to each of the tip portions of a pair of electrodes while the discharge lamp is in the lighting state, and thus the state in which the distance between the electrodes is short can be kept.
However, in this drive method, there arises a problem that there occur the blackening, the devitrification, and so on of the discharge lamp main body to thereby reduce the lifetime of the discharge lamp.
Further, JP-A-2010-123478 (Document 3) discloses a lighting device of alternately supplying a pair of electrodes with an alternating current having a steady lighting frequency in a range of 60 through 1000 Hz and an alternating current having a lower frequency in a range of 5 through 200 Hz, which is lower than the steady lighting frequency, to thereby light the discharge lamp.
However, in the lighting device described in Document 3, since both of the alternating current with the steady lighting frequency and the alternating current with the lower frequency have too low frequencies, there arises a problem that there occur the blackening, the devitrification, and so on of the discharge lamp main body to thereby reduce the lifetime of the discharge lamp. Further, although the projection is provided to each of the tip portions of the pair of electrodes while the discharge lamp is in the lighting state, the electrodes are gradually consumed while the discharge lamp is lit for a long period of time such as several hundreds or several thousands of hours, and it becomes unachievable to grow the projections to a predetermined size.